A Creature of the Soil

There are many things that can be said to constitute success in life, but one, surely, is the creation of a scientific cottage industry. If you can make your creation accessible to everyone through your writing, so much the better—that puts you in the pleasant company of Julian Huxley, C. P. Snow and Lewis Thomas. John Tyler Bonner has been working with and writing about the cellular slime molds (now usually referred to by the more artful term social amoebae) since he returned from World War II. Early in his career Bonner might have found some other creature to study, but the social amoebae have a certain aesthetic that attracted him and those of us who have followed.

These are organisms with a fascinating repertoire of behaviors. Perhaps the world champs of phagocytosis, they consume extraordinary quantities of bacteria, and their cycle of development excites wonder. They move—alone and as a group. But they usually go unseen, except by microbe hunters studying organisms of the soil.

If you want to observe the life cycle of the social amoebae, you’ll need a Petri dish with a rather poor medium, a pinch of soil and a few bacteria (Klebsiella or Escherichia coli, for example). Suspend the soil in water and let the large pieces settle. Mix the remaining liquid with some bacteria and spread it on the Petri dish. (These amoebae are usually to be found in the decaying leaf litter in soil, but they are surprisingly ubiquitous, so the liquid will undoubtedly contain some.) Overnight, a lawn of bacteria will form on the Petri dish. Ignore any fungi or nematodes and watch the lawn. After a few days, holes will start to appear in it as amoebae emerge from the slime mold spores that were in the soil and begin eating the bacteria.

Extraordinary sights will follow. As they feed, the amoebae will divide in two, their population doubling in size every few hours. When they run out of food (having eaten all the bacteria), the amoebae will begin to gather, arriving at a central collection point, one by one at first and then in great streams as thousands of them form an aggregate that is large enough to be visible to the naked eye. As they move, the amoebae will lengthen and attach to one another, the front end of one apposed to the hind end of another. You won’t tire of watching them.

Soon a little nipple will form on the aggregate, leading it to elongate and form a sluglike structure that very slowly migrates across the Petri dish. In another 10 to 12 hours, the slug will form a fruiting body with a ball of spores on top of a dead stalk. The movement of cells within the slug to create the fruiting body is so elegant a ballet that it seems choreographed. The process even has a title fit for a ballet: culmination. And yes, it has been put to music.

The first cellular slime molds to be identified were Dictyostelium mucoroides, a species discovered in 1869 by Oskar Brefeld, one of the greatest mycologists of the 19th century. Brefeld (who was one of the first to make pure cultures of a microorganism by streaking out fungal spores on gelatin plates) originally thought that these amoebae fused to form a syncytium (a cell-like structure filled with cytoplasm containing many nuclei), as do Physarum, one of the acellular slime molds. Later, however, he recognized that the amoebae remain individuals, and he corrected himself. The field languished until the 1930s, when Kenneth Raper isolated the model organism Dictyostelium discoideum and began to work out the biology of its development. Raper’s discoveries—that a slug is dominated by its anterior end, and that each half of a slug that has been cut in two has the capacity to regenerate—are elegantly described in Bonner’s book.

Bonner’s contribution came when he determined that the mechanism that the amoebae use to get together to form a multicellular organism is chemotaxis. He found that one cell releases an attractant. Its chemical nature was initially unknown, so he called it acrasin (after Acrasia the witch, who attracted men in Edmund Spenser’s The Faerie Queene). The acrasin attracts other amoebae, and these then also release acrasin in relay fashion to attract amoebae behind them to form a large aggregate in the center of a collection territory. It was eventually discovered that different species emit different attractants; the acrasin for D. discoideum is 3’-5’-cyclic adenosine monophosphate, or cyclic AMP. Bonner tells readers that this discovery was made in his lab by others while he was away, and that all he did was to pay for it from his grant, but he is being characteristically modest.

By now a lot is known about Dictyostelium because the genus presents a number of experimental advantages, including genetic tractability. This book will tell you more about the history of the early discoveries—with amusing anecdotes—than about the molecular biology of these organisms. The text is heavy on Bonner’s evolutionary and ecological perspective—things that modern graduate students tend to ignore but probably should not. The molecular biology is adequately referenced here, but you will find it more thoroughly explained elsewhere.

Studies of these intriguing organisms over the years focused at first on their development, that extraordinary production of fruiting bodies by starving cells. Some of the mechanisms required for that—chemotaxis, cell motility and the great complexity of the cytoskeleton—have made Dictyostelium a supreme contributor to our knowledge in those areas. More recently, other research topics along the lines of Bonner’s evolutionary and ecological interests have been approached with Dictyostelium. How is it that genetically different amoebae can cooperate to form a fruiting body in which 20 percent of them die as stalk cells? One would think that evolution would select variant amoebae that cheat and say “thanks, but no thanks” to being a dead stalk cell rather than a spore. In the lab and in the wild, these organisms have projected us into the interesting world of sociobiology, and we’ve been left scratching our heads a bit about how this cooperative life cycle came to be. There are other new areas: Who would have thought that these amoebae would prove to be perfect targets for some of the worst pathogenic bacteria—Legionella pneumophila, Vibrio cholerae, Mycobacterium marinum—and would thus provide an avenue to study those pathogens, a new cottage industry?

From this book, amateurs will learn some of the history of Dictyostelium and experts will enjoy being reminded of that history. If you are a lover of the diversity of biological life explained with style, then this book will be a good read for you. Even better, it would be an entertaining gift for any teenagers you know who are intrigued by science. They would not be the first young people to be encouraged in science by one of Bonner’s books.

Richard H. Kessin, a professor of pathology and cell biology at Columbia University, is the author of Dictyostelium: Evolution, Cell Biology, and the Development of Multicellularity (Columbia University Press, 2001).